The present invention relates generally to a circuit and method for accurately measuring the internal impedance of a battery to permit improved accuracy in determining the xe2x80x9cremaining run-timexe2x80x9d of a device powered by the battery without disrupting the operation of the device.
It is well known that the calculation of xe2x80x9cremaining run-timexe2x80x9d of a battery powered device is dependent on accurate knowledge of the internal impedance of the battery. The voltage drop across the internal impedance of the battery or of an individual cell of the battery decreases the actual amount of power that can be delivered by the battery to the battery powered device at a particular current level before the battery voltage falls below an acceptable xe2x80x9csafexe2x80x9d level specified by the battery manufacturer. Also, the amount of power that can be dissipated in the internal battery impedance decreases the amount of remaining run-time that can be provided by the battery to the battery powered device.
It also is well known that the internal impedance of the battery increases substantially as the battery ages, and more specifically, it is known that the internal impedance of the battery increases with battery xe2x80x9cagexe2x80x9d much more rapidly than the battery storage capacity decreases with age of the battery. For example, the battery storage capacity typically fades by only a few percent during the same period of time in which the battery impedance doubles. Battery impedance varies as much as +xe2x88x9215 percent even in newly manufactured battery cells. The battery impedance also depends strongly on temperature (about 1.5 times the change with 10 degree Centigrade change of temperature) and increases by a factor of roughly 2 during first ⅕ of battery life, but the amount of this increase can not be precisely predicted. All of these elements of uncertainty make the estimation of remaining battery capacity based on battery voltage under load very imprecise and no longer satisfactory for the rapidly growing reliability needs of users of battery powered mobile devices. None of the known xe2x80x9cbattery monitorxe2x80x9d circuits and methods are capable of making accurate correction for the increase in internal battery impedance with age. This causes indications of remaining run-time of a device powered by an xe2x80x9cagedxe2x80x9d battery to be very inaccurate, and the error is especially large if the battery is supplying power to a high-power device that fully discharges the battery in less than two hours.
Note that the xe2x80x9cagexe2x80x9d of a battery is generally understood to refer more to the number of charge/discharge cycles to which the battery has been subjected than to the actual amount of time that the battery has existed.
One of the prior techniques for taking into account the change of the internal battery impedance during a determination of remaining run-time of a device powered by the battery includes making a numerical correction based on stored database information that characterizes the dependence of the internal battery impedance on cumulative charge that has passed through the battery or the number of charge/discharge cycles to which the battery has been subjected. This technique has the disadvantage that a battery monitor circuit being used is dependent on the particular type of battery being monitored. Implementation of such a battery monitor circuit requires time-consuming collection of internal battery impedance change data during prolonged charge/discharge cycling of the battery. Even if the capability and time for providing such a database are available, this method is quite imprecise because the change in internal impedance of a battery is heavily dependent on various battery operating conditions, such as temperature, charge/discharge cycling frequency, etc. Furthermore, any implementation of a battery monitor that can effectively utilize such a database is undesirably complex and expensive.
Another prior art technique for taking into account the change of the internal battery impedance during a determination of remaining run-time of a device powered by the battery includes use of an external frequency generator to produce an excitation signal that is applied to the battery and measuring the response of the battery output voltage to the excitation signal. However, this technique is not well-suited to battery monitors in mobile applications because the noise level of the voltage to be measured is very high compared to the level of the excitation signal resulting from normal operation of the battery powered device, causing inaccurate measurement. Also, the use of the external signal generator results in additional power consumption and added complexity of the battery monitor design.
Thus, there is an unmet need for a circuit and method capable of accurately indicating remaining run-time of a battery and device powered by the battery, which method and circuit are much less dependent on the xe2x80x9cagexe2x80x9d of the battery than is the case for the prior art.
There also is an unmet need for a circuit and method which avoid inaccuracies in determinations of present battery capacity, usable battery capacity, usable energy, and/or remaining run-time due to use of inaccurate estimates of internal battery impedance.
There also is an unmet need for a circuit and method which avoid inaccuracies in determination to present battery capacity, usable battery capacity, usable energy, and/or remaining run-time caused by frequent load switching.
It is an object of the present invention to provide a circuit and method for accurately indicating remaining run-time of a battery and device powered by the battery, which method and circuit are quite independent of the xe2x80x9cagexe2x80x9d of the battery.
It is another object of the invention to provide a circuit and method which avoid inaccuracies in determinations of usable battery capacity, usable energy, and/or remaining run-time, and to a slight extent, present battery capacity, due to use of inaccurate estimates of internal battery impedance.
It is another object of the invention to provide a circuit and method which avoid inaccuracies in determination to present battery capacity, usable battery capacity, usable energy, and/or remaining run-time caused by frequent load switching.
It is another object of the invention to provide a circuit and method which allows precise estimation of internal battery impedance as a criterion of battery health for diagnostic purposes without disconnecting battery from the device it is powering and without interference with device operation.
Briefly described, and in accordance with one embodiment, the present invention provides a method and system for automatically determining the internal impedance of a battery (30) by operating a processor (13) to analyze current flowing through the battery to determine if a transient condition due to change of current is occurring and determining when the transient condition has ended. A voltage of the battery is measured while a steady current is being supplied by the battery. The present depth of discharge (DOD) of the battery is determined and a database is accessed to determine a corresponding value of open circuit voltage. The internal impedance is computed by dividing the difference between the measured voltage of the battery and the open circuit voltage at the present DOD by an average value of the steady current. A remaining run-time of the battery can be determined using a total zero-current capacity of the battery, integrating the current to determine a net transfer of charge from the battery, determining a value of total run-time that would be required to reduce the voltage of the battery to a predetermined lower limit, determining the duration of the integrating, and determining the remaining run-time by subtracting the duration of the integrating from the total run-time.
In one described embodiment, an internal impedance (R(DOD)) of a battery (30) is computed by operating a processor (13) to analyze current flowing through the battery to determine if a transient condition due to change of current is occurring and to determine when the transient condition has ended. A voltage of the battery (V(DOD)) is measured while a steady current (I) is being supplied by the battery. The present depth of discharge (DOD) of the battery is determined and a database stored in a memory is accessed to determine a value of open circuit voltage (V_OCV(DOD)) of the battery for the present DOD. The internal impedance is computed by dividing the difference between the measured voltage of the battery (V(DOD)) and the value of the open circuit voltage (V_OCV(DOD)) at the present DOD by an average value (I_average) of the steady current (I). The database is updated using computed values of the internal impedance.
In one embodiment wherein a small but measurable current I_small flows through the battery during the voltage measurement, the measured voltage is corrected by the value of a voltage drop across the internal resistance of the battery due to the small but measurable current I_small flowing through the battery according to the expression
V0corr=V0xe2x88x92R(DOD0,T)*I_small,
wherein DOD0 is subsequently recalculated using V0corr instead of V0.
In one embodiment, a remaining run-time (t_rem) of the battery by obtaining or using a previously obtained value of a present total zero-current capacity Qmax of the battery, integrating the current (I) to determine a net transfer of charge from the battery, determining a value of total run-time (t_total) that would be required to reduce the voltage of the battery to a predetermined lower limit (Vmin), determining the duration of the integrating, and determining the remaining run-time (t_rem) by subtracting the duration of the integrating from the total run-time. The determining of the total run-time (t_total) includes operating the processor to utilize the database to determine the value of total run-time which causes the expression
OCV(DOD0+t*I/Qmax,T)xe2x88x92I*R(DOD0+t*I/Qmax,T)
to be equal to the predetermined lower limit (Vmin), wherein T is the battery temperature, I is the current flowing through the battery, and R(DOD0+t*I/Qmax, T) is the internal resistance of the battery, wherein DOD0+t*I/Qmax is the DOD value at time t, and the database includes a section with parameters defining the dependence R(DOD,T) of the internal resistance as a function of DOD and T. The determining of the total run-time includes performing an iterative computing process to determine a value of total run-time (t_total) that causes the expression OCV(DOD0+t*I/Qmax,T)xe2x88x92I*R(DOD0+t*I/Qmax,T) to be equal to the predetermined lower limit (Vmin) for a constant power condition or a constant internal battery resistance condition.